Every now and then, a Web site comes along that makes you wonder why it didn’t exist years ago. That’s how I felt when I learned about ZocDoc.com – an online booking system that lets you search for a doctor nearby who can see you right away.

Calling your doctor and scheduling an appointment three days in the future could be a thing of the past. Of course, it also means you might need to see a doctor who doesn’t know your name. Does that matter, though, when you’re sick and eager to get better?

Here are three reasons why a ZocDoc IPO (if there’s even one on the horizon) might be a good buying opportunity:

1) Investors are already knocking on ZocDoc’s doors. ZocDoc’s one of several companies that were identified as “rising stars” on SecondMarket – a site where users can buy shares in private companies (see our post How to buy stock on SecondMarket for more). While ZocDoc doesn’t have any shares available on SecondMarket yet, it’s one of the top three social sites SecondMarket users want the opportunity to invest in (behind only Pinterest and Practice Fusion).

2) Rapid growth. ZocDoc’s already got 1.2 million users a month in 17 markets (per CNBC), and the company plans to roll out nationwide next year. ZocDoc’s pushing for rapid growth (rather than an early IPO) to try to get a head start on copy-cat entrepreneurs, CEO Cyrus Massoumi told CNBC. An IPO would presumably come after the nationwide rollout.

3) Big backers. ZocDoc has already raised $95 million since launching in 2008. $50 million came from well-known venture capitalist firm Digital Sky Technologies. What’s really interesting, though, is the source of another $25 million. Apparently, it came directly from Goldman Sachs (per Beta Beat). That’s an nontraditional move that shows Goldman really likes what they see at ZocDoc.

4) A viable business model. Unlike a lot of tech start-ups, ZocDoc has a well-defined, easy-to-understand business model: the company pulls in $250 a month for every doctor that’s listed on its site. Should ZocDoc firmly establish itself as the online leader in its niche, it would have the sort of steady cash flows most websites dream about.

One of my favorite parts about the company is that Massoumi – the company’s co-founder – seems to understand that it needs to stay laser-focused on doing one thing and doing that one thing very well.

“We have one of the best lists of practicing physicians in the country right now,” Massoumi told Venture Beat. “That is an incredibly valuable rolodex, and people are always asking us to partner with them on that data. But we believe our success comes from our single minded focus, and we’re sticking to that.”

Ignoring short-term gains at the expense of doing one thing very well is the key to success that we’ve seen at companies like Instagram and Pinterest. ZocDoc understands that, and my guess is it’ll help them make the transition from start-up to a company with true staying power.